As a method for developing an electrostatic charged image in an electrophotography and the like, a dry development method is employed frequently. In such a dry development method, a toner is charged usually as a result of the friction with a carrier such as an iron powder, glass beads and the like, and then deposits on an electrostatic latent image on a photoconductor via an electric attractive force, and then is transferred onto a paper and fixed by a heat roller, whereby establishing a permanent visual image.
As a fixation method, a heat roller method is employed frequently in which a toner image on a sheet to be fixed is passed with being brought into contact under pressure with the surface of the heat fixation roller with its surface is made from a material having a toner releasing ability.
In this heat fixation roller method, a toner capable of being fixed at a further lower temperature is desired for the purpose of improving the efficiency of the cost such as power consumption as well as increasing the reproducing speed.
However, in an attempt to improve the low temperature fixation performance described above, problems are encountered such as an offset phenomenon tends to be experienced in which a part of the toner deposits on the surface of the heat fixation roller and subsequently transferred on a paper, or a blocking phenomenon in which the toner tends to be aggregated by the heat exerted to respective resins via various environments.
To solve such problems, Japanese Patent No.2988703 proposes using, as a binder resin for toner, a crystalline polyester resin containing terephalic acid and C2-C6 straight alkylene glycol-derived units in an amount of 50 mole % or more based on the total monomer units employed.
However, this technology suffers from a difficulty in maintaining a high temperature offset-resistance and an anti-blocking performance without affecting the lower temperature fixation performance adversely, since it employs only a crystalline polyester resin which narrows the range of the temperature enabling the fixation.
Japanese Patent No.2704282 proposes using, as a binder resin for toner, a non-crystalline polyester resin obtained by polymerizing a trivalent or higher valency-carrying monomer, an aromatic dicarboxylic acid and an aliphatic alcohol containing a branched chain-carrying aliphatic alcohol in an amount of 50 mole % or more.
However, this technology suffers from an insufficient low temperature fixation performance since it employs only a non-crystalline polyester resin.
Japanese Kokai Publication Hei-4-97366 and Japanese Kokai Publication Hei-4-313760 propose, as a toner exhibiting an excellent balance of the low temperature fixation performance and the high temperature offset-resistance, a toner containing as toner resins two types of the polyesters having different softening points.
However, the compatibility between these two polyesters is not sufficient, and the polyester having a lower softening point allows the blocking to occur easily, and deposits onto the fixation roller to allow the filming to occur easily, and a disadvantageously reduced transparency of the resin due to the poor compatibility is also encountered.
Japanese Kokoku Publication Hei-5-44032 proposes using, as a binder resin for toner, a block copolymer of a low melting point crystalline polyester and a high melting point crystalline polyester.
However, this technology involves a problem of difficulty in obtaining a transparent resin since the binder resin becomes an opaque resin.
Also since a blocking phenomenon is readily experienced when a toner is exposed to a temperature higher than the glass transition point of the toner resin, the development of a toner polyester resin, which hardly undergoes a blocking phenomenon is also under way. As a toner polyester resin which hardly undergoes a blocking phenomenon although its low temperature fixation temperature is not so low, a polyester resin having a specific composition which gives a satisfactory effect is disclosed in Japanese Kokai Publication Hei-4-337741, while Japanese Kokai Publication Hei-10-36490 describes that it is effective to specify the composition of the polyester resin, and to limit the glass transition point within the range from 45 to 70° C.
However, even when using the toner resins of these technologies, the problematic blocking phenomenon is experienced upon exposure of the toner to a temperature around the glass transition point of the toner resin, although the blocking phenomenon at ambient temperature is less encountered.